Accurate amplitude measurement of multi-GHz AC signals and multi-Gbps random data patterns is challenging, especially if the measurement must be performed in a short time and without a sampling clock phase aligned to the incoming data signal. Conventional methods for amplitude measurement typically rectify or square the incoming signal to produce a DC component whose value represents signal amplitude. The DC component of the rectified or squared signal is extracted using a low-pass filter and is measured by an analog-to-digital converter or a simple comparator. Such approaches suffer from two main drawbacks: first the low-pass filtering of the rectified output can take a relatively long time to settle to an accurate value in response to amplitude changes; second, the accuracy of such approaches requires the use of rectifier or squarer stages that run at very high power to provide effective bandwidths well above the maximum input signal frequency. There is therefore a need for accurate, efficient, means of measuring the amplitude of high-speed signals.